billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

Anything from 20mm to 24mm should do it for you. Make sure your flash can zoom out that wide, most can. Good luck and keep on shooting until the end.
Or course you could stack them for a tighter grouping and then you can use a 28mm lens easily.
It also depends on how far back you can stand from them. If you have the room a 28-35 should do the trick.

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

Avoid using a very wide angle lens - people at the edges will not be happy with the results. Better to use a normal focal length and stitch a pano. It's an old technique, but just as valid today as it was years ago, and far easier with the digital tools we have.

https://photoapps.expert/tips/2017/1/13/group-portrait-composite-lightroom-photoshop#.WgcBFVuPLCM

unless you are bringing a lot of lighting, try for good natural light, perhaps on the shady side of a building, under the shade of some trees, or if you are lucky, you'll get a cloudy day.

And to avoid people with awkward expressions and closed eyes, tell the group to close their eyes, and on the count of three they are to open them. Then take your shot. They will generally have appealing expressions, be focused on you, and few, if any will have their eyes closed.

wanderingbear Loc: San Diego

Suggestion. Since it sounds like you are in the same area as the location of the building in which you will have the photo shoot. Would it be a good idea to go over there at the same time of day and see what the light really is. Natural, florescent, etc. Kind of guess the size of the potential group and set up accordingly, risers ?? The flash distance is another matter, effective distance.

Good luck. The Wanderingbear

Tracyv Loc: Del Mar, Ca

I recently got the Tamron SP 15-30mm f/2.8 Di VC USD for my Nikon D750, I just love this lens.

canon Lee

Just to make a point about "on camera flash"; The light width and power is only effective up to 10' at best. This means close distance which narrows the coverage, especially if you use a diffuser which reduces the flash out put by 1 stop. Stacking is not the best look. The best look you will get with a flash would be harsh and unevenly lit... I would encourage the OP to do the shoot right by having the correct amount of light to accommodate proper shooting distance and a lens of 24mm~104mm! Shooting outdoors would be another option to look into.

imagemeister Loc: mid east Florida

I would prefer a lens no wider than 23mm on APS to avoid distortions. I would prefer a high camera viewpoint - maybe 8-10 feet high. This angles the camera downward keeping the plane of focus more even across the faces. I would prefer using the even-ness of strong available lighting with a strong near camera flash for catchlights in the eyes......